In circulating fluidized bed combustion/gasification systems which utilize coal and biomass as raw material, the bed material (inert ash/sand) inside the reactor is required to be recycled after it rises through the reactor and is captured by the cyclone. Solid circulation inside the conventional systems may not be stable while the said process is being carried out, because the fluidized bed pressure barrier (loop seal) may lose its function upon being affected by pressure fluctuations inside the reactor. When the loop seal loses its function, the bed material inside the loop seal escapes from the cyclone and thus the pressure barrier between the downcomer and the reactor is broken. In this case, it is necessary to shut down and restart the system in order to recharge the system with new bed material.
In circulating fluidized bed systems, it is necessary to prevent the reverse flow of gases from reactor to the downcomer, in order to get stable solid circulation. Pressure barrier is usually provided in order to prevent reverse gas flow, which can be fluidised bed or static sand/ash column. Besides, in conventional circulating systems, 6-9% and 12-18% of total primary air is supplied to the loop seal to fluidize the pressure barrier bed material in combustion and gasification systems, respectively. Air provided from thereof is supplied with more pressure than the primary air and therefore efficiency of system decreases.
The Great Britain patent document no. GB 1291343, an application in the state of the art, discloses a solid fuel combustion system wherein pulverized coal is burned in combustion chamber with fluidized bed. Fly ash and burnt gases are fed to the cyclone separator mounted outside the combustion chamber and fed to the fluidized bed region again by means of the conduit, wherein the ejector is placed on unburnt particles, which are separated in the cyclone. Thus, the ejector prevents blockage of the conduit or malfunctions due to increase of pressure in the fluidized bed region.
The Japanese patent document no. JP62190309, an application in the state of the art, discloses a combustion system with bubbling fluidized bed. In the system disclosed in the said document, gases, which occur after combustion reactions taking place in the combustion region with fluidized bed, can contain some particles. The gas moving to the upper regions of the reactor together with the particles is dragged to the second combustion region of the reactor together with the mixture of air fed from the ejector and the fine particles kept in the cyclone. By means of the ejector system operating as an air curtain, retention ratio of the particles, which are being dragged upwards, increases inside the reactor and the amount of the particles reaching the cyclone decreases.
The United States patent document no. U.S. Pat. No. 4,730,563, an application in the state of the art, discloses a fluidized bed combustion system. In the said system, the gases occurring in the combustion region with fluidized bed move to the upper regions of the reactor together with the particles. The aim is that the centrifugal particle separator placed in the outlet region of the reactor primarily captures coarse particles. Particles separated by the particle separator are sent to bottom region of the fluidized bed by means of a conduit. In the outlet region of this conduit, ejector is used in order to overcome the counter pressure in the bed region. Combustion air pressurized by compressor is used as propellant gas in the ejector.
The United States patent document no. U.S. Pat. No. 4,882,400, an application in the state of the art, discloses a polymerization system with fluidized bed. In the said system, particles leaving the fluidized bed region together with the carrier gas are separated from the gas to a large extent by means of cyclone separator. Particles separated in the cyclone region enter the reactor again, just over the fluidized bed region, by means of the ejector. Pressurized carrier gas is used as propellant gas in the ejector.
The Japanese patent document no. JP10148468, an application in the state of the art, discloses a drying system with a circulating fluidized bed. In the said system, the mixture of intense particles and drying gas in the drying region with fluidized bed is separated from each other to a large extent by means of cyclone separator. Particles separated in the cyclone region are collected in the particle storage tank. Particles collected in the tank enter the drying system with fluidized bed again by means of ejector.
The United States patent document no. U.S. Pat. No. 6,269,778, an application in the state of the art, discloses a combustion system with circulating fluidized bed wherein circulation efficiency of fine particles is enhanced. In the said system, a part of fine particles escaping from the cyclone in conventional systems with loop seals, is captured.
The Japanese patent document no. JP2003038948, an application in the state of the art, discloses a particle processing system with bubbling fluidized bed. In the said system, a large part of the particles reaches the return conduit by separating off from the gas flow while passing through the spiral-shaped conduits before the reactor exit. The particles, which are separated by the spiral separator are sent to the bed region again by means of ejector system placed onto the return conduit in order to overcome the counter pressure in the bed region.
The United States patent document no. US2010190935, an application in the state of the art, discloses a polymerization system with fluidized bed. The said system overcomes the problem of blocking, which is experienced in current systems used for sending the fine particles into the reactor again, after they are captured in the cyclone. In order to overcome this problem, vertically oriented ejector is used instead of horizontally oriented ejector used in conventional systems. The propellant gas flow direction, the mixture of particle/gas flow direction and the outflow direction are same.
The Russian patent document no. RU2202069, an application in the state of the art, discloses a system and method for burning solid fuels. In the method disclosed in the said document, thermal decomposition process is performed in two stages, At the first stage, heating and partial thermal decomposition take place outside the fluidized bed. At the second stage, a vortex is created inside the fluidized bed and by means of the vortex created, the solid is separated from its ashes and burnt. In the said system, the ejector is connected to an air supply source.
The Chinese patent document no. CN103285785, an application in the state of the art, discloses a spouted fluidized bed reactor and a polymer preparation method. The system disclosed in the said document comprises a cyclone separator which is connected with a straight cylinder section and a liquid ejector. In the method disclosed in the said document, the gas sprayed from the ejector and the auxiliary gas are distributed equally and it is ensured that there is no aggregation in the reactor.